Infections due to the fungus Cryptococcus neoformans occur disproportionately in patients with impaired delayed-type hypersensitivity (DTH) responses. This proposal focuses on defining how the normal DTH response to C. neoformans activates leukocyte populations to inhibit and kill the fungus, and how fungal virulence factors help the organism elude host defenses. There are three major specific aims of this proposal. (1) Killing of C. neoformans by stimulated PBMC will be examined. Incubation of human peripheral blood mononuclear cells (PBMC) with killed C. neoformans results in the emergence of the ability of the cultured PBMC to kill C. neoformans. By fractionating PBMC into purified subpopulations, the cell phenotypes actually mediating killing will be defined. The contribution of cytokiens to the generation of fungicidal effector cells will be analyzed by assaying effector cell activity in the presence of purified cytokines or neutralizing antibodies to cytokines. (2) Cryptococcal binding and antigen presentation by bronchoalveolar macrophages (BAM) will be investigated. Preliminary data have established that human BAM, as opposed to other phagocytes studied, bind unopsonized C. neoformans, and that this binding is profoundly inhibited by component(s) in serum of apparent MW > 100K. The nature of this binding will be explored and purification and characterization of the factor(s) in serum that inhibit binding will be attempted. The ability of BAM to act as accessory cells supporting lymphocyte proliferation in response to C. neoformans will be analyzed. (3) Binding of IL-2 to C. neoformans and the biological consequences of that binding will be studied. Preliminary data have established that C. neoformans and other fungi bind IL-2, and that the bound IL-2 can stimulate mononuclear cell proliferation. The location and specificity of the binding, and its immunological consequences, will be determined. The effect cryptococcal capsule has on generation of the cellular responses outlined in the above specific aims will be explored by comparing virulent, encapsulated cryptococcal strains to a capsular mutants. The role specific opsonins play will be determine by comparing C. neoformans strains that are unopsonized versus those that are opsonized with complement, anticapsular antibody, or both. Completion of these studies over the next 5 years should contribute important new information to our basic understanding of cryptococcosis, and may provide a rational basis for studies in patients.